Optical modulators are used in optical communication systems to modulate an optical signal with an electrical signal. The electrical signal can have a high frequency, for example a few gigahertz. The modulated optical signal is propagated over large distances in an optical fiber. At the receiver end, the signal is detected using a photodetector, and the electrical signal is restored for further processing or transmission.
Presently, wavelength-tunable transmitters find an increasing application in optical networking systems capable of providing bandwidth on demand. Wavelength-tunable transmitters are also used as “any-wavelength” backup transmitters in fixed-wavelength transmitter arrays. A tunable transmitter requires a multi-wavelength optical modulator, the optical performance of which does not vary with wavelength and does not change, or changes negligibly, as the tunable transmitter ages.
Every optical modulator has a transfer curve, which represents a relationship between an amplitude of electrical signal applied and a magnitude of optical modulation obtained at the output of the optical modulator. Performance of many optical modulators depends on a choice of a set point, that is, a point on the transfer curve corresponding to zero modulating electrical signal. The set point can be adjusted by adding a DC signal to the modulating electrical signal, or by segmenting the optical modulator and applying the DC signal to one of its segments.
The set point of an optical modulator has a tendency to drift with temperature. To reduce thermal drift, a dither voltage is added to the modulating voltage, and a synchronous (lock-in) detection is employed to stabilize the set point. Referring to FIG. 1, a typical stabilized modulator system 100 is presented. The system 100 includes a Mach-Zehnder (MZ) optical modulator 102, a dithering unit 104, a lock-in detector 106 having a photodetector 107, a mixer 108 for mixing modulation and dithering signals 112 and 114, respectively, and a beamsplitter 110. In operation, an optical signal 116 is applied to the MZ optical modulator 102. The optical signal 116 is modulated with the modulation signal 112 applied to the MZ optical modulator 102 through the mixer 108. The dithering unit 104 generates the dithering signal 114, which is mixed by the mixer 108 into the modulation signal. A small fraction of an output optical signal 118 is directed by the beamsplitter 110 to the photodetector 107. The lock-in detector 106 generates a DC bias signal 120 based on a synchronously detected component of the output optical signal 118 at the frequency of the dithering signal 114.
Various modifications and adaptations of the stabilized modulator system 100 have been disclosed. By way of example, Tipper in U.S. Pat. No. 7,555,226 discloses an automatic bias controller for a MZ optical modulator. The automated bias controller of Tipper uses a microprocessor for both dithering and processing of the modulated optical signal. An optical power detector is used for detecting optical power of light emitted by one of two output arms of the MZ modulator. The detected signal is analyzed, and a bias voltage is adjusted so as to stabilize the set point of the MZ modulator. The other output arm of the MZ modulator is used to output the modulated optical signal.
Nahapetian et al. in U.S. Pat. No. 7,729,621 disclose a controller of a bias voltage for a MZ modulator, programmed to receive a dither signal, determine a derivative and/or an integral of the dither signal, and control a bias voltage for the MZ modulator based on the derivative and/or the integral of the dither signal.
Noguchi et al. in U.S. Pat. No. 7,561,810 disclose a bias controller of an optical modulator, wherein a pilot tone is added to the biasing voltage of the optical modulator. A monitor signal is branched into a signal path and a noise path. A notch filter is used in the noise path to suppress the pilot tone. The signals in both paths are synchronously detected, and the synchronously detected noise is subtracted from the synchronously detected signal to improve the signal-to-noise ratio.
Optical modulator control systems of the prior art are not adapted to control a modulator operating at different wavelengths. Accordingly, it is an object of the present invention to provide a control system, a modulator, and a tunable transmitter usable therewith, that can maintain optimal optical performance at a plurality of wavelengths, over extended periods of time.